So, in the very first page of IaWA is stated that you can play with "three or four of your friends"; however, you deal only four cards at the start of each session. So it's one character per player, one for the GM... and the fifth player? What does he do?

Oh, it's not one card per player. 4 cards will give you plenty of characters to go around.

Like, I'll draw some cards...

9H: The wetnurse of an austere scholar's adopted child.JS: A knowledge-mad sorceress, delving into ancient secrets.2C: A scholar and antiquarian, unmindful of danger.10D: The young, beautiful wife of an old man whom the gods have touched.

There we have the wetnurse, the scholar, the adopted child, the sorceress, the scholar and antiquarian, the young beautiful wife, the old man, and some number of gods. Plenty of characters!

Oh, it's not one card per player. 4 cards will give you plenty of characters to go around.

Like, I'll draw some cards...

9H: The wetnurse of an austere scholar's adopted child.JS: A knowledge-mad sorceress, delving into ancient secrets.2C: A scholar and antiquarian, unmindful of danger.10D: The young, beautiful wife of an old man whom the gods have touched.

There we have the wetnurse, the scholar, the adopted child, the sorceress, the scholar and antiquarian, the young beautiful wife, the old man, and some number of gods. Plenty of characters!